1. Field of the Invention
The present invention relates to an improved method for magnetic resonance (MR) applications, such as imaging, employing spatially localized magnetic field gradients and, more specifically, it relates to a method for reducing artifacts in an MR image acquired by employing a multi-polar readout scan technique such as echo planar imaging (EPI) and, most specifically, it is particularly advantageous in reducing ghost artifacts in an echo planar image. The invention also relates to an improved MR imaging apparatus employing spatially localized magnetic field gradients and, more specifically, it relates to such an apparatus for reducing artifacts in an MR image acquired by employing a technique such as EPI.
2. Description of the Prior Art
As is known to those of skill in the magnetic resonance imaging art, the gradient pulse sequence for an echo planar imaging (EPI) scan comprises a train of gradient pulses of continually alternating polarity in the readout direction, and a train of brief accompanying pulses or "blips" in the phase encoding direction. The EPI scan produces a corresponding train or series of gradient echoes comprising successive MR signals. The echoes are usefully designated as being "negative" or "positive" according to their respective positions in the echo train, since negative and positive echoes respectively correspond to readout gradients of negative and positive polarities.
Interleaved EPI is an ultra-fast imaging technique important for applications that desire high time resolution or short total acquisition times. EPI offers high speed for rapid imaging applications, especially those needing either high temporal resolution or short total acquisition times. See P. Mansfield, Multi-planar image formation using NMR spin-echoes, Journal Physics C, vol. 10, pp. 55-58, 1977; and U.S. Pat. Nos. 5,168,228; 5,287,854; 5,336,999; and 5,672,969.
A wide variety of applications take advantage of the speed performance of EPI, such as: (1) functional imaging, see K. Kwong, Functional magnetic resonance imaging with echo planar imaging, Magnetic Resonance Quarterly, vol. 11, pp. 1-20, 1995; (2) diffusion weighted imaging, see, R. Turner et al., Single-shot diffusion imaging at 2.0 Tesla, Journal of Magnetic Resonance, vol. 86, pp. 445-52, 1990; (3) perfusion imaging, see, M. Moseley et al., Ultrafast magnetic resonance imaging: diffusion and perfusion, Canadian Association of Radiologists Journal, vol. 42(1), pp. 31-38, 1991, and M. Stehling et al., Echo-planar imaging: magnetic resonance imaging in a fraction of a second, Science, vol. 254, pp. 43-50, 1991; and (4) interleaved EPI of various organs of the body, see, K. Butts et al., Echo-planar imaging of the liver with a standard MR imaging system, Radiology, vol. 189(1), pp.259-64, 1993 (liver), and G. McKinnon, Ultrafast interleaved gradient-echo-planar imaging on a standard scanner, Magnetic Resonance in Medicine, vol. 30(5), pp. 609-16, 1992 (heart).
An entire image can typically be acquired in times ranging from 50-200 ms.
Unfortunately, EPI is prone to significant ghosting artifacts, resulting primarily from system time delays that cause data matrix misregistration. In typical applications, system time delays are orientation dependent, resulting from anisotropic physical gradient delays. The behavior of time delays is characterized in oblique coordinates and the ghosting artifacts are caused by anisotropic delays.
Misregistrations of raw k-space data (i.e., k.sub.r, readout direction; k.sub.p, phase encoding direction; and k.sub.s, slice direction) caused by system time delays and phase offsets are a significant problem for sequences such as EPI and gradient and spin-echo (GRASE), see, K. Oshio et al., GRASE (gradient- and spin-echo) imaging: a novel fast MRI technique, Magnetic Resonance in Medicine, vol. 20(2), pp. 344-49, 1991, where lines of k-space are acquired in two directions.
System filters have causal impulse responses which manifest as k-space misregistrations. See, A. Oppenheim et al., Discrete Time Signal Processing, pp. 250-52, 313, and 314, 1982; and H. Bruder et al., Image reconstruction for echo planar imaging with non-equidistant k-space sampling, Magnetic Resonance in Medicine, vol. 23, pp. 311-23, 1992.
Gradient hardware also contributes to system delays, as do demodulations, RF coils, and other sources that are patient independent.
"Reference scans" are often used to measure time delays and phase offsets as part of the imaging protocol. See, H. Bruder et al., Image reconstruction for echo planar imaging with non-equidistant k-space sampling; A. Jesmanowicz et al., Phase correction for EPI using internal reference lines, 12th Annual Meeting of SMRM, Book of Abstracts, p. 1239, 1993. However, reference scans can add significant time to a magnetic resonance (MR) examination, especially if the prescription or imaging plane is changing orientation and time delays are orientation dependent. This increases the total examination time and defeats the primary advantage of EPI, namely, speed. Furthermore, repeated reference scanning is not feasible with EPI based real-time MR systems, because scan planes are continually changing.
Anisotropic delays cause a ghosting artifact resulting from alternating k-space shifts in the phase encoding direction. These shifts in k-space caused by anisotropic delays bias time delay measurements made with reference scans in oblique coordinates. For this reason, EPI techniques that rely on reference scan measurements preferably avoid these measurements in oblique coordinates.
Cardiac imaging, in particular, poses several challenges for EPI. First, the field inhomogeneities present in the vicinity of the heart are accentuated by the heart's proximity to the lungs in the mediastinum, as well as susceptibility effects from large veins carrying deoxygenated blood. See, S. Reeder et al., In vivo measurement of T*.sub.2 in human hearts at 1.5 T: implications for cardiac echo planar imaging, Magnetic Resonance in Medicine, vol. 39, pp. 988-98, 1997.
In addition, ventricular motion and ventricular blood flow can cause significant phase shifts in the imaging signal. The geometry of the thorax itself can change between breath-holds, which may also present problems for EPI reference algorithms that are object dependent.
Once time delay estimates are made, post-processing algorithms are often used to correct for these delays. These algorithms require additional computational expense.
Many EPI systems rely upon post-processing routines to correct for time delays and constant phase offsets. This is usually done by applying the principles of the Fourier shift theorem. After Fourier transformation in the readout direction, each profile is multiplied by the appropriate phase roll and constant phase offset, before Fourier transformation in the phase encoding direction. While such post-processing algorithms can be effective and accurate only when the gradient waveform delays are isotropic, a more attractive and computationally less expensive approach is to modify the gradient waveform to produce data that does not require correction even with anisotropic gradient waveform delays.
It is known, for a single oblique non-orthogonal scan plane orientation, to employ amplitude correction values for only the phase encoding gradient waveform to, thus, avoid use of post-processing algorithms to make k-space misregistration corrections in only the phase encoding direction. See, X. Zhou et al., Reduction of a new nyquist ghost in oblique echo planar imaging, In Fourth Annual Meeting of ISMRM, p. 1477, 1996; U.S. Pat. No. 5,672,969. In this manner, the sampled data is free from misregistrations in only the phase encoding direction.
Gradient hardware systems are the only orientation dependent source of system hardware delays. These delays can result from eddy currents, see, C. Ahn et al., Analysis of eddy currents in nuclear magnetic resonance imaging, Magnetic Resonance in Medicine, vol. 17, pp. 149-63, 1991, as well as sequencer errors that delay the intended waveform. Regardless of their source, these delays can be modeled as a time shift of the entire gradient waveform.
The incremental delay is independent of slice orientation when the delay from each of the different gradient amplifiers is the same. If these delays are different, however, the effective delay becomes image orientation dependent. Such anisotropy has serious implications for echo planar image reconstruction, especially for real-time EPI systems that rapidly change image orientation and require rapid image reconstruction and delay compensation, without repeated reference scans.
Time shifts of gradient waveforms can only correct anisotropic delays to the precision of the gradient waveform sequencers. For example, if the sequencer precision is 4 .mu.s, then delay corrections can only be made to the nearest 2 .mu.s. At a bandwidth of .+-.62.5 kHz, this represents a 0.25 sample shift, which can cause considerable ghosting. See, S. Reeder et al., Quantification and reduction of ghosting artifacts in interleaved echo planar imaging, Magnetic Resonance in Medicine, vol. 38, pp. 429-39, 1997.
Once time delay calibration of a scanner has been performed, there are several steps that can be taken to correct for the system hardware delays. The easiest way to correct for orientation independent delays from within the pulse sequence is to delay the sampling period to better align echoes in the readout direction of k-space. As well, anisotropic gradient delays can be compensated by shifting entire physical gradient waveforms by -t.sub.x, -t.sub.y, and -t.sub.z, for the x, y and z physical gradients. This latter correction is only possible, however, in pulse sequences that calculate and play physical waveforms, and do not rely upon oblique rotation boards. In any case, simple shifts of the gradient waveforms and sampling periods can only correct delays to the precision of the gradient waveform sequencers.
For these reasons, there remains a very real and substantial need for an improved apparatus and method of operation thereof for magnetic resonance imaging.